The Quaternary slip rate of the Yangsan Fault offshore the SE Korean Peninsula and implications for seismic hazard assessment

The Yangsan Fault, a NNE-trending strike-slip fault, is a pronounced active structure in the Korean Peninsula which recently generated an earthquake with moment magnitude M_W 5.5 (M_L 5.8) that was the largest instrumentally recorded event in Korea. High-resolution seismic profiles obtained on a dense grid reveal deformation of Quaternary sequences that indicates the offshore extension of the Yangsan Fault to the SE continental shelf of the Korean Peninsula. The offshore fault system comprises a main strand and subsidiary Riedel shears formed by dextral slip. We estimated the Quaternary lateral slip rate of the Riedel shear zone using seismic profiles. The Quaternary slip rate of 0.73 ± 0.07 mm/yr was estimated from the dextral displacement of the base boundary of the Quaternary sequences along the main strand. Modelling of fault activities based on empirical relations incorporating the slip rate indicates that (1) the offshore Riedel shear zone is capable of generating earthquakes with magnitude up to M_W 6.07–6.25 with recurrence intervals of 550–1500 years and (2) the individual Riedel shears can generate earthquakes with magnitude less than M_W 5.87.